In conventional television systems, such as "PAL", "NTSC" and "SECAM", the brightness of a displayed image is conveyed by the direct current (D.C.) level of the base-band video signal luminance component. To provide a uniform frame of reference for expressing "brightness" and other video components (such as the horizontal synchronizing component) it is customary to refer to base-band luminance signal components in terms of a "normalized" signal range rather than specific voltage levels. One common normalized range is expressed in terms of so-called "IRE" units after a standardization organization (the Institute of Radio Engineers) that proposed it. Briefly, the standard IRE signal range extends from a low value of -40 IRE, which represents the horizontal synchronizing signal "tip" level, to high value of +100 IRE which represents the luminance signal peak white level. Within the range between -40 IRE and +100 IRE there is a level associated with video signal "black level".
In the United States, prior to the adoption of color transmission standards, the luminance signal "black level" was defined to be "0" IRE thus providing a black-to-white range of exactly 100 IRE for monochrome video signals. This standard changed, in the United States, upon the adoption of compatible color television transmission standards. Specifically, in the U.S. color television transmission standard, the peak white level and the sync tip levels were maintained at levels of +100 IRE and -40 IRE, respectively. However, the "black level"0 was changed from 0 IRE to +7.5 IRE. This new IRE level is generally referred to as the black level or the "black set-up" level. The zero IRE unit level continued to be used as the blanking reference level of the horizontal blanking interval and is some times referred to as the "blacker than black" level.
Under this new standard, the signal range between -40 IRE (sync tip level) and zero IRE ("blacker than black" or blanking) signal level is used for horizontal synchronizing signal components and the range between +7.5 IRE (the new "black level" or the so-called "set-up" level) and 100 IRE (peak white) represents signals used for conveying active video information representative of displayed images. See, for example, the article IRE STANDARDS ON TELEVISION: DEFINITIONS OF COLOR TERMS, 1955, Appendix B entitled "Full text of FCC Technical Standards Amended to Incorporate Color", which is believed to have been published by the IRE (Institute of Radio Engineers) circa 1955.
To summarize the foregoing, under the NTSC color transmission standard, the range between zero IRE level and +7.5 IRE level represents, in effect, a "guard zone" between the synchronizing components and the active video components of a video signal. Advantageously, in countries adopting the new transmission standards, it is easier to distinguish between the synchronizing luminance signals (e.g., -40 IRE to zero IRE for horizontal sync) and the active video signals (e.g., +7.5 IRE to +100 IRE) because they occupy different signal ranges.